Recording materials in which the light-sensitivity of diazo compounds is utilized are roughly divided into three types. One of these types is known as a wet developing type recording material, in which a support is laminated with a sensitive layer containing a diazo compound and a coupling component as the main components. In this case, an original is superposed on the recording material and, after exposure, development is carried out in an alkali solution. The second type of recording material is known as a dry developing type, in which, unlike the first type, development is carried out with ammonia gas. The third type of recording material is known as a heat developing type, which is further divided into several sub-types, such as: a type in which its sensitive layer contains an ammonia gas generating agent like urea that generates ammonia gas when exposed to heat; a type in which its sensitive layer contains an alkali salt of an acid such as trichloroacetic acid that loses its property as an acid when exposed to heat; and a type in which a higher fatty acid amide is used as a coloring auxiliary, and a diazo compound and a coupling component are activated by heat melting, etc.
Because of the use of a developing solution, the wet developing type material has several disadvantages, such as problems from a maintenance point of view due to the troublesome steps needed for supplementing and discharging the solution, the necessity of large scale equipment and the like, as well as the production of a moist recording material just after copying which requires time to dry for writing.
The dry developing type material also has some problems. For example, it requires that the developing solution to be replenished, by analogy with that with a wet developing type. Also, it requires a gas absorption apparatus to absorb generated ammonia gas, thereby increasing the overall size of the instrument. In addition, it produces an ammonia smell immediately after copying.
On the other hand, unlike the cases of the wet and dry developing types, the heat developing type recording material has an advantage from maintenance point of view because it does not require a developing solution. The prior art heat developing type material, however, requires the use of expensive instrumentation, because it requires a high developing temperature of from 150.degree. to 200.degree. C. Moreover, it is necessary to control the temperature within about .+-.10.degree. C. to prevent under-development and color tone changes. In addition, because of such a high developing temperature, the diazo compound to be employed in the material should have a high heat resistance. These heat-resistant compounds, however, are disadvantageous in many cases from the viewpoint of the formation of a high color density.
Many attempts have been made to produce a low temperature development system (90.degree. to 130.degree. C.), but these attempts have suffered from the disadvantage of lowered shelf life of the recording material. Consequently, in spite of its advantage from a maintenance point of view when compared to the wet developing type and the dry developing type, the heat developing type recording material has not yet been used as a main diazonium-based recording system.
For the purpose of obtaining a desired color density by heating a recording material comprising a support having thereon a sensitive layer containing a diazo compound and a coupling component, it is necessary to develop the color quickly by the rapid melting, dispersion and reaction of the components when heated. For this purpose, it is preferable that the reaction system be under a basic condition, because such a condition has the effect of enhancing the reaction. Consequently, it is essential to include a basic compound in a coating layer for the purpose of preparing a light- and heat-sensitive recording material having a recording speed increased to such an extent as not to impede seriously its practical use by low temperature heating.
Another essential factor for a light- and heat-sensitive recording material is to prevent coloring on the surface of the material during storage before copying and to prevent a decrease in the developed color density.
In order to prepare a light- and heat-sensitive recording material having such a good shelf life and a quick recording speed, several attempts have been made as described above, but with no practical results.
When a recording material is designed in such a manner that sufficient color development with a high color density can be obtained at a low heating temperature, it is very likely that a coloring reaction occurs even during storage of the material at room temperature prior to its copying. Such a reaction causes coloring on the surface of the recording material, which should be maintained white until its use. Such a coloring problem is especially serious in the case of a red (magenta) color developing type recording material, because even a slight coloring on the surface (fog) becomes visible due to the high visual sensitivity of the color.
With the aim of overcoming such seemingly incompatible problems, the inventors of the present invention have conducted intensive studies and found that the formation of fog can be prevented by preparing a recording material comprising a support having thereon a thermally developable sensitive layer containing a diazo compound, a coupling component, and a basic compound, wherein the diazo compound is included in microcapsules. The inventors have further continued their studies in terms of screening basic compounds, processes for the preparation of microcapsules, and the like, and have succeeded in preventing coloring on the surface of the recording material during storage before copying, as has been disclosed in JP-A-2-54251 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). As additional background for the present invention, the present inventors have also conducted studies on improvement of the storage stability and oil soluble property of coupling components and found that specific coupling components have excellent properties (U.S. patent application Ser. No. 07/718,476). The present inventors have further conducted intensive studies on the combination of the diazo compound with coupling components and found that a magenta color having a markedly excellent hue can be formed when the specific diazo compound is used in combination with a coupling component selected from the group consisting of the cyclohexane-1,3-dione derivatives disclosed in the aforementioned patent publication (JP-A-2-54251), the 3-cyclohexenone derivatives, and the 2-cyclohexenone derivatives. The present invention has been accomplished as a result of these efforts.